A number of modem devices and their associated data communication protocols are well known in the prior art. The rapid increase in internet access and the undesirably long downloading times typically associated with current internet use has spawned the development of new high speed modems that can transmit data files at theoretical speeds of up to 56 kilobits per second (kbps). The typical 56 kbps solution combines pulse code modulation (PCM) techniques with several existing features set forth in the standard V.34 data communications protocol. Although such a solution can lead to an increased data rate, the "combined" technology can present several technical problems that require consideration. Some of these problems are addressed in the following United States patent applications, which are incorporated herein by reference: "Spectral Shaping in Signal-Point Limited Transmission System," Ser. No. 08/746,731, filed Nov. 15, 1996; "System for Controlling and Shaping the Spectrum and Redundancy of Signal-Point Limited Transmission," Ser. No. 08/756,383; and "Method and Apparatus for Implementing Shell Mapping Techniques in the Context of a PCM-based Modem Communications System," Ser. No. 08/760,646, filed Dec. 4, 1996.
Linear equalizers are adaptive filters that are routinely utilized to compensate for intersymbol interference (ISI), which is a common practical impairment found in many transmission systems, e.g, voiceband and digital data modems. The severity of ISI depends upon the characteristics of the particular communication channel. For example, most data communication and modem systems use a single channel to transmit several symbols in succession. Unfortunately, an inherent effect of most communication channels is that a received message or symbol can be corrupted with interference from a neighboring (with respect to time) message or symbol. For a more detailed treatment of adaptive filters, see Simon Haykin, ADAPTIVE FILTER THEORY, ch. 4, 5, 8, and 9 (3d ed., Prentice Hall 1996), the subject matter of which is incorporated by reference herein.
Since ISI can be modeled as a linear pulse spread in time, it can be compensated for by a linear filter which "inverts" the pulse spreading by detecting the content of each message spread over time at the receiver. ISI can also be considered in the frequency domain; if a signal is subject to an uneven frequency attenuation, then ISI is introduced. Viewed in this manner, the equalizer corrects for the uneven frequency response by making the combined channel and equalizer response as flat as possible.
Adaptive linear equalizers may be subjected to training procedures that seek to optimize the equalizer transfer functions prior to the transmission of the actual data. Typically, equalizer training is accomplished by transmitting a known data sequence to the receiver, which utilizes the same data sequence as an input to an adaptation circuit. A correction signal may be used to adjust one or more filter taps resident in the linear equalizers.
In baseband systems, such as PCM modems, AC coupling inherent in the transmission channel can cause a form of ISI known as baseline wander. Baseline wander is proportional to the running digital sum (RDS) of the line code employed by the transmission system. Accordingly, conventional PCM modems often utilize line codes that minimize or regulate the upper limit of the RDS, thus controlling the detrimental effects of baseline wander. Such line codes tend to color the spectrum of the transmitted signal by reducing the DC content of the transmitted signal. For a general discussion of spectral control techniques, line codes, and related filtering, see Edward A. Lee & David G. Messerschmitt, DIGITAL COMMUNICATION, ch. 12 (2d ed., Kluwer Academic Publishers 1994), the subject matter of which is incorporated by reference herein.
Unfortunately, the use of line codes may adversely affect the performance of the linear equalizers employed by the receive modem. For example, the use of a colored training signal in a receiver that utilizes a decision feedback equalizer (DFE) may cause the DFE to settle at a local, rather than the global, minimum with respect to the optimum settings of the associated filter taps. In addition, a colored training signal generally results in smaller eigenvalues in the receive signal autocorrelation matrix. Accordingly, the use of a colored training signal, e.g., a training signal subject to line coding, can result in poorly adjusted linear equalizers at the modem receiver. Less than optimum initial settings can adversely affect the performance of the modem during actual data transmission.